The stripe on the back of a credit card is a magnetic stripe, often called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a plastic-like film. Each particle is really a tiny bar magnet about 20-millionths of an inch long.
Illustration by Rosaleah Rautert
Your card has a magstripe on the back and a place for your all-important signature.
The magstripe can be "written" because the tiny bar magnets can be magnetized in either a north or south pole direction. The magstripe on the back of the card is very similar to a piece of cassette tape (see How Cassette Tapes Work for details).
A magstripe reader (you may have seen one hooked to someone's PC at a bazaar or fair) can understand the information on the three-track stripe. If the ATM isn't accepting your card, your problem is probably either:
- A dirty or scratched magstripe
- An erased magstripe (The most common causes for erased magstripes are exposure to magnets, like the small ones used to hold notes and pictures on the refrigerator, and exposure to a store's electronic article surveillance (EAS) tag
Information on the Stripe
There are three tracks on the magstripe. Each track is about one-tenth of an inch wide. The ISO/IEC standard 7811, which is used by banks, specifies:
- Track one is 210 bits per inch (bpi), and holds 79 6-bit plus parity bit read-only characters.
- Track two is 75 bpi, and holds 40 4-bit plus parity bit characters.
- Track three is 210 bpi, and holds 107 4-bit plus parity bit characters.
Your credit card typically uses only tracks one and two. Track three is a read/write track (which includes an encrypted PIN, country code, currency units and amount authorized), but its usage is not standardized among banks.
The information on track one is contained in two formats: A, which is reserved for proprietary use of the card issuer, and B, which includes the following:
The format for track two, developed by the banking industry, is as follows:
- Start sentinel - one character
- Primary account number - up to 19 characters
- Separator - one character
- Country code - three characters
- Expiration date or separator - four characters or one character
- Discretionary data - enough characters to fill out maximum record length (40 characters total)
- LRC - one character
For more information on track format, see ISO Magnetic Stripe Card Standards.